1. Field of the Invention
The present invention relates to a current detecting circuit for detecting a load current supplied from a power supply portion to a load.
2. Description of the Related Art
In a current supply circuit for supplying a load current from a power supply portion to a load when switching means is turned on, fuse has been heretofore widely used as protection means against an overcurrent. The fuse is inserted in series to the circuit so that, when an overcurrent flows, the fuse is blown to cut off the circuit. Hence, the fuse has an advantage of being capable of protecting the circuit surely. However, when an overcurrent flows to make the fuse blown out, the circuit cannot be restored unless the blown fuse is exchanged for a new one. There is therefore a problem that much labor is required for restoration of the circuit. Particularly in a vehicle, the modularization of the current supply circuit has been advanced in recent years and such fuse has been incorporated in a module. To exchange an old fuse for a new one, the old fuse had to be taken out first from the inside of the module and the new fuse was then incorporated in the module. Hence, the efficiency in the work of exchanging the fuse for a new one was very low.
Therefore, a configuration to dispense with the fuse has been discussed as follows. That is, in the configuration, the current flowing in the current supply circuit is monitored so that, when the level of the current deviates from a normal range, the switching means is turned off to cut off the circuit. A current detecting circuit for detecting a load current accurately is required for achieving the configuration.
A circuit as shown in FIG. 7 is known as the current detecting circuit of this type. In the circuit shown in FIG. 7, a high-precision low resistor 103 is series-connected between a battery 101 and a load 102 and the current flowing in the low resistor 103 is converted into a voltage by a conversion circuit 104. On the other hand, a reference voltage is generated from a reference voltage generating circuit 105. The converted voltage is compared with the reference voltage by a comparator circuit 106 so that a judgment is made as to whether the current flowing in the low resistor 103 is abnormal or not.
Incidentally, the reference voltage generating circuit 105 generated such a reference voltage with the earth as reference. Hence, it was necessary that a voltage drop over the low resistor 103 was converted into a value with reference to the earth by the conversion circuit 104. It was however difficult that the voltage drop over the low resistor 103 was converted accurately into a value with reference to the earth by the conversion circuit 104. Moreover, there was a problem that the configuration of the conversion circuit 104 became complex.
In addition, a battery voltage in a car varies relatively largely even in the case where the battery operates normally. To make it possible to generate a constant reference voltage regardless of the variation of the battery voltage, there was another problem that the circuit configuration of the reference voltage generating circuit 105 became complex.
The present invention is devised to solve the aforementioned problems and an object of the present invention is to provide a current detecting circuit in which a load current supplied from a power supply portion to a load can be detected by a simple circuit configuration.
According to the present invention, in a current supply circuit for supplying a load current from a power supply portion to a load when switching means interposed between the power supply portion and the load is turned on, there is provided a current detecting circuit constituted by: a voltage generating circuit for generating and outputting a second voltage which is lower by a predetermined voltage value than a first voltage outputted from the power supply portion; a current detecting resistor interposed between the power supply portion and the load and having a predetermined resistance value; an analog-digital conversion circuit which operates on the basis of a potential difference between the first voltage and the second voltage to thereby convert an analog value into a k-bit (k is an integer not smaller than 2) digital value; and an arithmetic circuit for calculating the load current or a value corresponding to the load current on the basis of digital values which are obtained by analog-digital conversion of voltages respectively on one and the other ends of the current detecting resistor by the analog-digital conversion circuit when the switching means is turned on.
According to this configuration, the second voltage lower by the predetermined voltage value than the first voltage outputted from the power supply portion is generated and outputted from the voltage generating circuit so that the analog-digital conversion circuit is operated by the potential difference between the first voltage and the second voltage.
When the switching means is turned on, the voltage on one end of the current detecting resistor and the voltage on the other end of the current detecting resistor are converted into k-bit digital values respectively by the analog-digital conversion circuit. The load current or a value corresponding to the load current, for example, the voltage drop over the current detecting resistor, is calculated on the basis of results of the conversion.
The analog-digital conversion circuit is operated by the potential difference between the first voltage and the second voltage lower by the predetermined voltage value than the first voltage as described above. By serving the second voltage as a virtual earth voltage for the analog-digital conversion circuit, it is unnecessary to convert the first and second voltages into values with reference to the actual earth. Hence, the load current or a value corresponding to the load current can be calculated by a simple configuration.
Further, in accordance with this configuration, detection of the load current can be made accurately. For example, the predetermined voltage is 5 V under the condition of k=8, the resolution of the digital value is 5 V/255 which is near to 20 mV (5 V/255≈20 mV). That is, when the numerical value k is changed, detection of the load current can be made with accuracy of a desired degree.
Further, the analog-digital conversion circuit may include: a divisional voltage output circuit for successively selectively outputting summation values which are obtained by adding the second voltage to various voltage values which are obtained by multiplying a divisional voltage by m (m is an integer of from 0 to (n-1)), the divisional voltage being obtained by dividing a potential difference between the first voltage and the second voltage into (n-1) parts (n is an integer satisfying n=2k), and a comparator circuit for comparing the voltage on the one end of the current detecting resistor with each of the summation values which are successively selectively outputted from the divisional voltage output circuit and for comparing the voltage on the other end of the current detecting resistor with each of the summation values which are successively selectively outputted from the divisional voltage output circuit; and the arithmetic circuit may calculate the load current or a value corresponding to the load current on the basis of results of the comparison in the comparator circuit.
According to this configuration, by use of the divisional voltage output circuit, the potential difference between the first voltage and the second voltage is divided into (n-1) parts (n is an integer satisfying n=2k) to obtain divisional voltages. Voltages are obtained by multiplying the divisional voltage by a factor m (m is an integer of from 0 to (n-1)). Summation values obtained by adding the second voltage to the multiplied voltages respectively are successively selectively output. For example, the divisional voltage output circuit is configured to have (n-1) resistors each of which has an equal resistance value and which are series-connected between the output line of the power supply portion and the output line of the voltage generating circuit.
Further, when the switching means is turned on, the voltage on one end of the current detecting resistor is compared with each of the summation values successively selectively output. The voltage on the other end of the current detecting resistor is also compared with each of the summation values successively selectively output. A load current or a value corresponding to the load current, for example, a voltage drop over the current detecting resistor, is calculated on the basis of results of the comparison.
In this manner, each of the summation values obtained by adding the second voltage, which is lower by the predetermined voltage value than the first voltage, to a voltage obtained by multiplying the divisional voltage by a factor m is used as a reference voltage. Each of the voltage on one end of the current detecting resistor and the voltage on the other end of the current detecting resistor is compared with the reference voltage. Hence, because the second voltage serves as a virtual earth voltage, the voltages on the opposite ends of the current detecting resistor need not be converted into values with reference to the actual earth. Hence, the load current or a value corresponding to the load current can be calculated by a simple configuration.
Further, the arithmetic circuit may calculates so that, as a result of comparison by the comparator circuit, when sizes of the voltage on the one end of the current detecting resistor and each of the successively selectively outputted summation values are changed over each other, the summation value at that time is made to be the voltage value on the one end, while, when sizes of the voltage on the other end of the current detecting resistor and each of the successively selectively outputted summation values are changed over each other, the summation value at that time is made to be the voltage value on the other end, and, on the basis of a difference between the voltage values on the one and the other ends of the current detecting resistor, a voltage drop over the current detecting resistor is obtained.
According to this configuration, in the results of the comparison by the comparator circuit, when the sizes of the voltage on one end of the current detecting resistor and each of the summation values successively selectively outputted from the divisional voltage output circuit are changed over each other, the summation value is made to be the voltage value on one end of the current detecting resistor. When the sizes of the voltage on the other end of the current detecting resistor and each of the summation values successively selectively outputted from the divisional voltage output circuit are changed over each other, the summation value is made to be the voltage value on the other end of the current detecting resistor. A voltage drop over the current detecting resistor can be calculated on the basis of the potential difference between the voltage value on one end of the current detecting resistor and the voltage value on the other end of the current detecting resistor. Hence, the voltage drop can be detected with resolution of predetermined voltage/(n-1). Hence, when the numerical value n is taken large, the voltage drop can be detected accurately.
Further, the analog-digital conversion circuit may form a part of a CPU integrated on a semiconductor wafer, the CPU may have a power-supply input terminal and a ground terminal, and the first voltage may be applied to the power-supply input terminal and the second voltage may be applied to the ground terminal.
According to this configuration, the first voltage outputted from the power supply portion is applied to the power-supply input terminal of the CPU and the second voltage outputted from the voltage generating circuit is applied to the ground terminal of the CPU. Hence, the CPU operates with an operating voltage which is a predetermined voltage (for example, 5 V) to be the potential difference between the first voltage and the second voltage while the second voltage is used as a virtual earth voltage. Hence, the voltage drop over the current detecting resistor can be calculated accurately by a simple configuration.
Further, the current detecting resistor is formed from a semiconductor switching device constituting the switching means. If the predetermined resistance value is the resistance value of the ON-state resistance generated when the semiconductor switching device is turned on, the semiconductor switching device such as an FET can serve also as a current detecting resistor. Hence, the current detecting resistor need not be provided as a separate resistor, so that the circuit configuration can be simplified more greatly.